1. Field of the Invention
The present invention relates to an optical element used in various optical apparatuses.
2. Description of Related Art
Many optical elements, such as a lens, a prism, and a splitter, are used in image pickup systems and projection optical systems of optical apparatuses, such as a DSC (Digital Still Camera), a DVC (Digital Video Camera), a camera for a cellular phone, a projection television, a BD (Blu-ray Disk) player, a DVD (Digital Versatile Disk) player, and a CD player.
As the optical apparatuses have higher magnification, higher fineness, and higher brightness as well as smaller size and weight, the optical elements also are required to have improved accuracy, function, handlability, strength, and cost.
Typically, an optical element is made of glass or plastic. Generally, a grinding method is used when an optical element is produced from glass and an injection molding method is used when an optical element is produced from plastic (JP 62(1987)-85918 A), from the viewpoint of mass productivity and accuracy. In some cases, an optical element is formed by a precision glass molding method in which a glass lump weighed in advance is plasticized and thereafter pressed by an upper die and a lower die to be formed directly into a desired shape (JP 2000-53428 A).
JP 62(1987)-85918 A and JP 2000-53428 A disclose respectively an invention intended to enhance the releasability between a molded product (an optical element) and a molding die, and an invention intended to enhance the transfer of the shape of a molding die to a material. Specifically, a stepped engagement portion inclined with respect to an optical axis direction is provided on an outer peripheral surface of the optical element in order to allow the optical element to rotate when it is released from the molding die (JP 62(1987)-85918 A), and a gas discharging groove is formed in the molding die (JP 2000-53428 A).
Hereinafter, a method for producing an optical element by the precision glass molding method will be described.
FIGS. 9A to 9C are cross-sectional views illustrating each process of a conventional precision glass molding method. FIG. 9A is a cross-sectional view illustrating the starting phase of the molding. FIG. 9B is a cross-sectional view illustrating a phase during the molding. FIG. 9C is a cross-sectional view illustrating the completion phase of the molding.
First, as shown in FIG. 9A, an optical material to be molded 30 is set in a cavity formed by an upper die 31, a lower die 32, and a cylindrical die 33, and all of them as a whole are placed between a lower head 35 and an upper head 34. The lower head 35 and the upper head 34 have heating and pressing mechanisms.
Then, the upper die 31, the lower die 32, the cylindrical die 33, and the optical material to be molded 30 are heated using the upper head 34 and the lower head 35.
When the temperature of the optical material to be molded 30 reaches a desired temperature that allows the material 30 to be deformed, the optical material to be molded 30 is pressed by the upper head 34 as shown in FIG. 9B. The pressing deforms the optical material to be molded 30 into the shape of an optical element 11.
Thereafter, the optical element 11 is cooled while the temperature and pressure are adjusted so that an optically functional surface is transferred to the optical element 11 satisfactorily. When the temperature of the molded optical element 11 is lowered to a temperature that allows the optical element 11 to be taken out, the upper head 34 and the upper die 31 are raised to take out the optical element 11.
FIGS. 8A and 8B show the conventional optical element 11 molded by the above-mentioned method. FIG. 8A is a top view of the optical element 11 when viewed from an optical axis direction. FIG. 8B is a cross-sectional view taken along a plane including an optical axis A.
The optical element 11 includes an optically functional part 12 and an outer peripheral part 15 provided around the optically functional part 12. The optical element 11 is produced so that a thickness T of the outer peripheral part 15 uniformly is 0.5 mm.
Upon observation, the optical element 11 has cracks 18 in the outer peripheral part 15. Specifically, the cracks 18 occur in the vicinity of a boundary portion between the optically functional part 12 and the outer peripheral part 15. Even more cracks occur when the thickness T of the outer peripheral part 15 is 0.5 mm or less.
Conceivably, the cracks 18 occur because the outer peripheral part 15 is sandwiched between the upper die 31 and the lower die 32 and thereby the shrinkage of the optical element 11 in a radial direction during the molding is hindered.
Specifically, the shrinkage of the optical element 11 in the radial direction occurs when the optical element 11 is cooled in the phase shown in FIG. 9B. However, since the outer peripheral part 15 of the optical element 11 is sandwiched between the upper die 31 and the lower die 32 in the phase of FIG. 9B, the shrinkage of the optical element 11 in the radial direction is hindered. Furthermore, the outer peripheral part 15 has a poor strength because the outer peripheral part 15 itself has a small thickness. Conceivably, these are the reasons why the cracks 18 occurred in the outer peripheral part 15.
The cracks that thus occurred lower the yield in the production of the optical element. Moreover, when the cracks grow and break the optical element, the fragments of the broken optical element need to be removed, lowering the production efficiency.
Particularly, optical elements required to have a high field angle and a high magnification, such as an optical element used in a DSC, etc., are required to be thinner every year, and the thicknesses of the optical elements tend to be increasingly small. In light of this, the occurrence of cracks mentioned above affects significantly the realization of a thin optical element in the future.